Container spreaders are crane-operated devices used to transport freight containers between a dock and a ship berthed adjacent the dock. Spreaders are also found on container yard equipment and on other container handling equipment. Many freight containers are built according to internationally recognized standards such as ISO 668. Typically, two types of ISO standard containers are encountered: 20 and 40 foot nominal length containers.
ISO standard corner fittings are built into the four corners atop each container. ISO standard pickup devices on the spreader mate with the corner fittings. The pickup devices are hydraulically, pneumatically or mechanically actuated twist-lock devices located on the ends of slide frames which extend longitudinally from the spreader's main beams. The slide frames can be extended to lift a single 40 foot container or retracted to lift a single 20 foot container. If another set of twist-lock devices is provided in the centre of the main beams, then the spreader can simultaneously lift two 20 foot containers when the slide frames are extended. Spreaders having the latter capability are known as "twin twenty" spreaders.
If a single 40 foot container located beneath the spreader is to be lifted, the slide frames are extended to position the outer twist-locks over the container's top corner fittings. The spreader is then lowered onto the container so that the twist-locks engage the corresponding corner fittings on the container. The twist-locks are then actuated to lock them onto the corner fittings, thus firmly attaching the container to the spreader. The spreader and container are then lifted to move the container to a desired location, following which the twist-locks are released from the corner fittings so that the spreader can be raised away from the container and maneuvered into position over another container. The same sequence is used to move a single 20 foot container, the only difference being that the spreader's slide frames are retracted to position the outer twist-locks relative to the corner fittings of a 20 foot container.
Eight twist-locks are provided for configuration as a twin spreader: four twist-locks for each of the two containers. The additional four twist-locks are located centrally on the spreader. The slide frames are extended to position the outer twist-locks over the outer corners of the two containers, with the central twist-locks remaining over the inner corners. The spreader is then lowered onto the containers. Ideally, all eight twist-locks simultaneously engage the corresponding corner fittings on the containers. However, problems are often encountered because the two containers may not be perfectly aligned relative to the spreader. The twist-locks are fixed on the spreader and thus cannot be adjusted to compensate for container mis-alignment.
In particular, unless the two containers are carefully positioned within a frame provided beneath the spreader, they may be mis-aligned relative to the spreader. Such frames are not commonly used because they are relatively expensive, require increased labour to position the containers, and must be moved if the spreader moves to a new pickup location. Accordingly, container mis-alignment is often encountered. Depending upon the degree of misalignment, it may not be possible for the twist-locks to simultaneously engage both containers. Specifically, if the containers are mis-aligned by more than about .+-.1/2" it may only be possible for one of the two pairs of four twist-locks to simultaneously engage all four corner fittings on one of the containers. The operator often wastes time attempting to position the spreader so that all eight twist-locks will engage the two containers, only to give up in frustration and lift the containers one at a time. This wastes considerable time and defeats the purpose of the twin spreader.
A further problem termed "swaybacking" is encountered if two 20 foot containers are to be simultaneously lifted from the bed of a transport vehicle. The weight of the containers bends the vehicle bed downwardly, causing the top surfaces of the containers to form a shallow "v" instead of remaining level and coplanar with one another. When a twin spreader is lowered atop the two containers, the outer twist-locks may engage the outer corner fittings on the two containers, but the central twist-locks may not be able to protrude sufficiently far into the "V" recess to engage the inner corner fittings. This again wastes time by necessitating separate removal of each container from the vehicle, defeating the purpose of the twin spreader.
Another problem is encountered in lifting pairs of 20 foot containers from atop a stack of containers. As pairs of containers are stacked, cones are often placed beneath the inner corner fittings located on the bottoms of each container. The cones guide the placement of each layer of containers. Because cones are not usually placed beneath the containers' outer corners, the inner ends of the containers are raised slightly relative to the outer ends. As successive layers of containers are added to the stack, the cumulative effect of the cones beneath each layer causes the top surfaces of the uppermost containers to form a shallow " " instead of remaining level and coplanar with one another. When a twin spreader is lowered onto two containers atop a stack of such containers, the central twist-locks may engage the inner corner fittings on the two containers, but the outer twist-locks may not be able to protrude sufficiently far into the recesses left at the ends of each container to engage the outer corner fittings. Time is again wasted as each container is separately removed from the stack and the twin spreader's purpose is again defeated.
The present invention provides a self-adjusting alignment mechanism which compensates for container mis-alignment by allowing the four central twist-locks of a twin spreader to shift in three dimensions as they engage the container corner fittings, thus enabling simultaneous lifting of two containers which are mis-aligned by as much as .+-.3 inches.